Certain substances, when bound to the membrane of neurons and other excitable cells, behave as molecular probes of membrane potential. The optical properties of these molecules, notably absorbance and flourescence, vary in a linear fashion with potential, and may, therefore, be used to monitor action potentials and synaptic potentials in a large number of neurons at once, without the necessity of using electrodes. We propose to develop more sensitive indicators, and the technology associated with them, and to use these "molecular voltmeters" for optical recording of neuronal signalling in individual neurons in a simple invertebrate ganglion.